OFDM communications system

ABSTRACT

A communications system comprising a base station, and a plurality of terminals served by that base station which may include an ad-hoc network of terminals. Information from the base station or transmitting terminal is transported to the receiving terminals in the form of symbols in an orthogonal frequency division multiplexed format. Each terminal has means for measuring a channel dispersion and rate of phase variation corresponding to multipath signal reception for that terminal and for reporting that dispersion and variation measure to the transmitting station. In response to the channel dispersion and phase change measurements, the base station or transmitting terminal is arranged to provide, for groups of terminals having a similar channel dispersion, respective symbol and prefix lengths on the basis of the dispersion measurement of that group of terminals.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/185,133, filed on Jun. 27, 2002, entitled “OFDM COMMUNICATIONSSYSTEM”, which claims benefit of priority from U.S. ProvisionalApplication Ser. No. 60/359,973, filed Feb. 27, 2002. Those prior patentapplications are hereby incorporated by reference in their entireties asthough fully and completely set forth herein.

FIELD OF THE INVENTION

This invention relates to communication systems and in particular to anarrangement and method for providing OFDM (Orthogonal Frequency DivisionMultiplex) communication.

BACKGROUND OF THE INVENTION

A number of wireless communication systems employ OFDM (OrthogonalFrequency Division Multiplex) transmission as a means of communicationbetween a base station and a number of terminals served by that basestation. OFDM downlink transmission from the base station to theterminals provides a series of symbols spread over a large number ofcarrier frequencies. Each of those symbols is provided with a cycleprefix which needs to be of sufficient length for the preceding symbolto have decayed thus minimising the risk of intersymbol interference.

In a typical wireless communication, this prefix length needs to besufficient to overcome the effects of multipath transmission for thoseterminals that are relatively distant from the base station. It will beappreciated that multipath transmission can arise from reflections ofthe radio signal from objects adjacent the transmission path. Becausethese reflected signals reach their destination via paths that arelonger than the direct ‘line of sight’ path, the received signaleffectively comprises a number of identical signals having differenttime delays. The effect is referred to as channel dispersion.

In such a system, due account has to be taken of channel dispersioneffects, particularly for terminals that are remote from the basestation. In current systems a prefix length is chosen that willaccommodate the higher degree of channel dispersion for outlyingterminals. However, the ‘worst case’ prefix length that is necessary toprovide effective downlink communication with a distant terminal is farlonger than is necessary for communication with terminals close to thebase station.

OFDM with cyclic prefixes exploits the eigenfunctions of a multipathchannel and is therefore considered to be the most efficient signallingmethod in such environments. However there are technological issuesinvolving the cost of the terminal FFT DSP (fast Fourier Transformdigital signal processing) and the required stability of localoscillators. In fact, OFDM as currently implemented exploits thetechnology in a significantly less than inefficient fashion. This isbecause the duration of the cyclic prefixes is determined, as discussedabove, by the worst case environment. In turn this fixes the symbolduration and sub-carrier bandwidths. When the overall channel bandwidthis fixed, the size of the FFT processing is also determined. This istypically in the 1024-4096 point region which is far too high for atypical user.

In a typical OFDM communications system, the worst case channeldispersion may be seen only by 1% of the terminals and the averagechannel may have a dispersion value only 1% to 10% of the worst casedispersion. Thus, in the current design paradigm, 1% of the terminals inan OFDM communications system are forcing the remaining 99% of theterminals to have symbol durations 10-100 times larger than is strictlynecessary for adequate reception of signal transmissions. While thiscurrent method of system design has no significant impact onperformance, there is a very significant impact on cost. High stabilitylocal oscillators with low phase noise are mandated in supposedly lowcost terminals, and a much larger FFT (fast Fourier transform) is usedthan is ideally necessary which increases the DSP (digital signalprocessor) load and battery power consumption.

Another problem occurs in channels where the phase of the radio path isunstable due to Doppler shifts or other propagation anomalies. If thephase changes more than 10° or so during the duration of an OFDM symbolthen the FFT algorithm used in the receiver, which by design assumes aperfect static channel, is mismatched to the symbol and there is aconsequent loss of amplitude of each of the symbols in the frequencybins accompanied by the appearance of cross talk between the frequencybins. The combination of these effects reduces the signal to noise ratio(SNR) of the demodulated symbols and increases the probability of errorin the communication channel.

The minimization of the joint problems of channel dispersion and phaseinstability requires the selection of OFDM symbols which have anintermediate length and which must be selected by in situ measurementsin the channel as it is used.

OBJECT OF THE INVENTION

An object of the invention is to overcome or at least to mitigate theabove disadvantage.

A further object of the invention is to provided an improved arrangementand method for providing OFOM communication between a central or basestation and a plurality of terminals.

A further object of the invention is to provided an improved arrangementand method for providing OFDM communication between terminals in anad-hoc network.

A further object of the invention is to provide an improved base stationfor an OFDM communications system.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof communication between one or more base stations and a plurality ofterminals served by the base station(s), the method comprisingtransporting information from the base station(s) to the terminals inthe form of symbols in an orthogonal frequency division multiplexedformat, and providing each symbol with a respective prefix, and whereinthe symbol and prefix durations are arranged on the basis of therequirements of individual communication channels.

According to another aspect of the invention there is provided a methodof communication between one or more base stations and a plurality ofterminals served by the base station(s) and in which information fromthe base station(s) is transported to the terminals in the form ofsymbols in an orthogonal frequency division multiplexed format, themethod comprising: at each terminal, measuring a channel dispersioncorresponding to multipath signal reception for that terminal andreporting that dispersion measure to the base station(s), and, at thebase station(s), providing for each terminal respective symbol andprefix lengths appropriate to the dispersion measurement of that groupof terminals.

In a preferred arrangement, terminals with a given symbol and prefixlength are grouped together and transmitted to as a group. It ispreferred not to transmit OFDM symbols of differing durations at thesame time, i.e. in the same time slot, as they will becomenon-orthogonal. Thus, the transmission will preferably have a timedivision multiple access (TDMA) format.

The method may be embodied in or controlled by software in machinereadable form on a storage medium.

Advantageously, the prefix length is matched to the duration of themultipath in a channel. Typically, the useful symbol length is about tentimes the prefix length.

In a further embodiment, when the prefix needed is longer than thesymbol then the whole symbol itself is used several times to extend thesymbol length by an integer number of times.

According to another aspect of the invention there is provided acommunications system comprising one or more base stations, and aplurality of terminals served by the base station(s), in whichinformation from the base station(s) is transported to the terminals inthe form of symbols in an orthogonal frequency division multiplexed(OFDM) format, in which each terminal has means for measuring a channeldispersion corresponding to multipath signal reception for that terminaland for reporting that dispersion measure to the base station(s), and,in which the/each base station is arranged to provide, for groups ofterminals having similar channel dispersion, respective symbol andprefix lengths appropriate to the dispersion measurement of that groupof terminals.

According to another aspect of the invention there is provided a basestation for use in a communications system in which information from thebase station is transported to the terminals in the form of symbols inan orthogonal frequency division multiplexed (OFDM) format, the basestation comprising: means for receiving channel dispersion measurementsfrom terminals served by the base station, and means for providing, forgroups of terminals having similar channel dispersion, respective symboland prefix lengths appropriate to the dispersion measurement of thatgroup of terminals.

According to another aspect of the invention there is provided acommunications system comprising one or more base stations, and aplurality of terminals served by that one or more base stations, andwherein some of the terminals have formed an ad-hoc network in whichinformation from the base station or other transmitting terminals istransported to the terminals in the form of symbols in an orthogonalfrequency division multiplexed format, in which each terminal has meansfor measuring a channel dispersion and channel phase stabilitycorresponding to multipath signal reception for that terminal and forreporting that dispersion and phase variation measure to the basestation or transmitting terminal, and, in which the base station ortransmitting terminal is arranged to provide, for a group of terminalsrespective symbol and prefix lengths on the basis of the dispersion andphase stability measurement of that group of terminals.

According to another aspect of the invention there is provided a methodof operating a base station in an orthogonal frequency divisionmultiplex (OFDM) communication system comprising a plurality ofterminals served by the base station, the method comprising the stepsof:

-   -   i) Receiving an indication of channel dispersion associated with        each said terminal;    -   ii) classifying said terminals into groups of terminals having a        similar channel dispersion; and    -   iii) for each group of terminals having a similar channel        dispersion, generating on the basis of the dispersion        measurements respective symbols and prefixes having appropriate        lengths for transmission to the terminals of that group.

According to another aspect of the invention there is provided an OFDM(orthogonal frequency division multiplex) communications signal for useon a channel subject to channel dispersion between a base station and aterminal, said signal comprising a plurality of symbols each having aprefix, and wherein the symbol and prefix lengths are adjusted on thebasis of a measure of the dispersion on that channel.

According to another aspect of the invention there is provided a methodof communication between peers of a communication network such that thattransmitter and receiver are both mobile terminals disposed inrespective ad-hoc networks, in which the terminals are arranged toperform channel dispersion measurements, the method comprising: at eachterminal, measuring a channel dispersion associated with multipathsignal reception for that terminal, and providing for a group ofterminals of similar dispersion measurement respective symbol and prefixlengths on the basis of the dispersion measurement of that group ofterminals.

In a further embodiment of the invention a subset of the terminals mayalso inter-communicate as an ad-hoc network as well as to the basestation, while a further overlapping subset of terminals may communicateonly with each other. In this case the terminals which are transmittingto other terminals have the same properties as, and are functionallyequivalent to, a base station.

It will of course be appreciated that although the technique is ofparticular application to wireless communication systems, it is alsoapplicable to wired systems or to systems using optical transport.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention and the best known method ofputting the invention into practice will now be described with referenceto the accompanying drawings in which:

FIG. 1 is a schematic diagram of an OFDM wireless communication systemaccording to a preferred embodiment of the invention;

FIG. 2 is a schematic diagram of a terminal for use in the system ofFIG. 1;

FIG. 3 is a schematic diagram of a base station for use in the system ofFIG. 1

FIG. 4 is a schematic graphical representation of the multipathstatistics of the system of FIG. 1;

FIG. 5 illustrates a preferred method of determining a ratio ofinter-symbol interference (ISI) to carrier power; and

FIG. 6 illustrates a preferred method of determining variable prefixlengths.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring first to FIG. 1, this depicts in schematic form a wirelesscommunications system in which a base station 11 communicates with aplurality of terminals 12. In the system of FIG. 1, downlinkcommunications from the base station to the terminals are transported inan OFDM (orthogonal frequency division multiplex) format. Although onlyone base station is depicted in FIG. 1, it will be appreciated that thenetwork may include a plurality of base stations. Further, thefunctionality of one or more base stations may be provided by an ad hocgroup of terminals.

In the system of FIG. 1, transmissions from the base station 11 to themore remote terminals e.g. 12′ are subject to multipath or channeldispersion interference. In FIG. 1, a direct path 13 is shown from thebase station 11 to the terminal 12′, and a second indirect path 13 a isshown resulting e.g. from a reflection from a building or a vehicle (notshown). The difference in arrival times of the two signals at theterminal 12′ has the potential to cause channel dispersion interference.

Referring now to FIG. 2, each terminal 12 served by the base station 11is powered from a battery 10 or alternative power source and is providedwith a channel estimator 20, an OFDM processor 22, and voice processor23. The terminal may also incorporate data handling, text messaging andvideo processing facilities (not shown). The channel estimator 20determines the dispersion resulting from multipath signal reception forthat terminal and reports that dispersion measure, together with theterminal identity, via the terminal antenna 21 to the base station 11.In response to that measurement, the base station provides for groups ofterminals respective symbol and prefix lengths appropriate to thedispersion measurement of that group of terminals. Methods of channeldispersion measurement or channel estimation will be apparent to thoseskilled in the art.

Referring now to FIG. 3, this illustrates in schematic form theconstruction of a base station for use in the network of FIG. 1. It willbe appreciated that, for simplicity and clarity, FIG. 3 shows only thoseparts of the base station as are necessary for the understanding of theinvention.

In the base station of FIG. 3, channel information 32 received from theterminals via receive antenna 31 is processed by OFDM control 33 whichclassifies the system terminals into groups, the terminals of each grouphaving a similar channel dispersion measurement. The OFDM processor 34provides, in response to the information received from the OFDM control33 appropriate symbol and prefix lengths for the groups of terminalsserved by the base station via transmit antenna 35.

In our system the set-up for the OFDM modulation is determined by theOFDM processor 34 from the terminal channel dispersion measurementsreceived at the base station via the Channel information 32 and OFDMcontrol 33 by the following method steps:

-   -   (i) determine the maximum dispersion of the channel T₀.    -   (ii) choose an acceptable energy loss 6 dB such that the cyclic        prefix uses some fraction of the available carrier power. If the        prefix has 10% of the symbol duration the loss is around 10        log₁₀(0.9)−½ dB.    -   (iii) Set the prefix length T_(P)=T₀ and the symbol duration        T_(S) to about 10T_(p).    -   (iv) Given a channel bandwidth B calculate the number of        independent samples BTs in the symbol    -   (v) Choose a power of 2 such that N=2^(m)≧BT and use an FFT        (fast Fourier transform) of size N for the OFDM modulation    -   This calculation is now related to the statistical nature of the        channel multipath. FIG. 4 shows the cumulative multipath power        and residual power at a given terminal for the downlink path.        For simplicity, this cumulative distribution S(d) is modelled as

$\begin{matrix}{{S(d)} = {1 - {\exp( \frac{d}{T_{0}} )}}} & (1)\end{matrix}$

-   -   with probability density distribution D(d)

$\begin{matrix}{{D(d)} = {\frac{1}{T_{0}}{\exp( {- \frac{d}{T_{0}}} )}}} & (2)\end{matrix}$

-   -   The inter-symbol interference (ISI) power in the symbol is upper        bounded by the integral of the tail of D(d) from T_(P) to ∞ as        illustrated in FIG. 4. For unit carrier power the ISI energy        E(T_(P)) which falls within the symbol is given by

$\begin{matrix}{{{E( T_{p} )} \approx {\frac{1}{T_{0}}{\int_{t = T_{P}}^{\infty}{{\exp( {- \frac{t}{T_{0}}} )}\ {\mathbb{d}t}}}}} = {\exp( {- \frac{T_{P}}{T_{0}}} )}} & (3)\end{matrix}$

-   -   while the carrier energy is given by        C(T _(S))=T _(S)  (4)    -   thus the carrier to ISI ratio is given by

$\begin{matrix}{{{CNR}( {T_{P},T_{S}} )} \approx \frac{T_{S}}{\exp( {- \frac{T_{P}}{T_{0}}} )}} & (5)\end{matrix}$

Fixing T_(S)=9T_(P) gives

$\begin{matrix}{{{SNR}( T_{P} )} = {9T_{P}{\exp( \frac{T_{P}}{T_{0}} )}}} & (6)\end{matrix}$

If the modulation, ranging from QPSK (Quadrature Phase Shift Keying) to64 QAM (Quadrature Amplitude Modulation), requires a signal to noiseratio (SNR)=γ dB, then the equation can be solved for T_(P). Table 1below shows T_(P) versus γ for some ITU standard channels. These are theoutdoor to indoor and pedestrian A and B models which have rms values of45 ns for 40% of the time and 750 ns for 55% of the time and thevehicular A and B which have rms dispersions of 370 ns for 40% and 4000ns for 55% of the time.

TABLE 1 Table 1: Duration of prefix (ns) vs rms dispersion to achieve adesired SNR γ T_(p) vs T₀ (dB) T_(p)/T₀ T₀ = 75 ns T₀ = 370 ns T₀ = 750ns T₀ = 4000 ns 0 0.1 7.5 37 75 400 5 0.27 20.25 99.9 202.5 1080 10 0.6145.75 225.7 457.5 2440 15 1.13 84.75 418.1 847.5 4520 20 1.8 135 6661350 7200 25 2.6 195 962 1950 10400 30 3.5 262.5 1295 2625 14000

There are two features to note from Table 1. The first is the shortduration of the prefix required to provide moderate signal to ISI ratiossuch as coded QPSK or Turbo coding in the range 0 to 5 dB. The second isthe large prefix required to provide the higher values of γ which wouldbe required for 64 QAM (−30 dB) in a highly dispersive channel. We haveprefix lengths ranging from 7.5 ns to 14 μs, a ratio of 2×10³. Thisdemonstrates that the conventional “one size fits all” prefix strategyis in some difficulty when combined with variable QAM in highlydispersive channels.

Table 2 below shows the FFT size to achieve the required SNR's onaverage in the given channels.

TABLE 2 Table II: FFT size to achieve γ dB SNR on average in thestandard channels γ (dB) T₀ = 75 ns T₀ = 370 ns T₀ = 750 ns T₀ = 4000 ns0 1 2 4 32 5 2 8 16 64 10 4 16 32 128 15 4 32 64 256 20 8 32 64 512 2516 64 128 512 30 16 64 128 1024

From the data provided in Table 2, it can be concluded that OFDM isusually overdesigned. The high dispersion (750 ns) outdoor to indoor andpedestrian channel which is thought to model the HSDPA channel onlyneeds a 128 point FFT on average, even to run 64-QAM at 30 dB SNR. Inparticular, I have found that, by reserving the longer prefixes forthose few users who really need them, the symbol lengths can be reducedsubstantially.

In a preferred embodiment, the adaptive OFDM operates by choosing theFFT length for the average channel and varying the prefix length to suitthe individual user as is illustrated in FIG. 5. This allows a reductionof the DSP load for all terminals but causes some slight loss incapacity for the infrequent high dispersion channels.

The prefix duration can be determined by feedback of the channel impulseresponse from the terminal to the base station. Advantageously, when theprefix needed is longer than the symbol then the whole symbol itself maybe used several times to extend the symbol length by an integer numberof times.

The dilemma of finding an optimum prefix for both types of channel canbe circumvented in a further embodiment by providing two classes ofterminals; one low cost class with short FFTs and low cost localoscillators designed for portable use; the second class being asemi-portable terminal with the normal OFDM long FFT and high stabilitylocal oscillators. Short FFTs are also required when the channel phasesuffers from instability or there are high Doppler shifts due toterminal mobility. These two classes of terminal are interoperable withall base stations by suitable signalling.

In a further embodiment, an ad hoc group of mobile terminals may operatetogether to provide the functional equivalent of a base station, e.g. toform a microcell within a cell. Terminals within such a group maycommunicate directly with each other or with a system base station.

It will be understood that the above description of a preferredembodiment is given by way of example only and that variousmodifications may be made by those skilled in the art without departingfrom the spirit and scope of the invention.

The invention claimed is:
 1. A wireless terminal for use in a wirelesscommunications system, the wireless terminal comprising: a channelestimator configured to determine a channel dispersion associated withmultipath signal reception and to report the channel dispersion to abase station in communication with the wireless terminal; an orthogonalfrequency division multiplex (OFDM) unit configured to receive from thebase station OFDM symbols having a symbol length and a prefix lengththat is based upon the channel dispersion reported to the base stationby the wireless terminal and other wireless terminals having a similarchannel dispersion; wherein, the OFDM unit is configured to receive anOFDM symbol in which the OFDM symbol is retransmitted an integer numberof times in response to a determination by the base station that arequired prefix is longer than the symbol.
 2. The wireless terminal ofclaim 1, wherein the symbol length has a length such that the phase ofthe channel can be considered to be stable.
 3. The wireless terminal ofclaim 1, wherein the symbol length is substantially equivalent to tentimes the prefix length.
 4. The wireless terminal of claim 1, whereinthe base station corresponds to a group of terminals operatingcollectively as the base station.
 5. The wireless terminal of claim 1,wherein the channel estimator is further configured to measure a phasestability of a receive channel and to report the phase stability to thebase station.
 6. The wireless terminal of claim 5, wherein the OFDM unitis configured to receive an OFDM symbol having a symbol duration that isshort enough that the channel is considered to be static for theduration of each symbol.
 7. A method comprising: a wireless terminaldetermining a channel dispersion associated with multipath signalreception; the wireless terminal reporting the channel dispersion to abase station in communication with the wireless terminal; the wirelessterminal receiving from the base station orthogonal frequency divisionmultiplex (OFDM) symbols having a symbol length and a prefix length thatis based upon the channel dispersion reported to the base station by thewireless terminal and other wireless terminals having a similar channeldispersion; wherein the receiving includes receiving an OFDM symbol inwhich the OFDM symbol is retransmitted an integer number of times inresponse to a determination by the base station that a required prefixis longer than the symbol.
 8. The method of claim 7, wherein the symbollength has a length such that the phase of the channel can be consideredto be stable.
 9. The method of claim 7, wherein the symbol length issubstantially equivalent to ten times the prefix length.
 10. Anon-transitory computer readable storage medium including instructionsexecutable by a processor to: determine a channel dispersion associatedwith multipath signal reception and to report the channel dispersion toa base station in communication with the wireless terminal; receive fromthe base station OFDM symbols having a symbol length and a prefix lengththat is based upon the channel dispersion reported to the base stationby the wireless terminal and other wireless terminal having a similarchannel dispersion; and further receive an OFDM symbol in which the OFDMsymbol is retransmitted an integer number of times in response to adetermination by the base station that a required prefix is longer thanthe symbol.
 11. A base station for use in a wireless communicationssystem, the base station comprising: a channel information unitconfigured to receive from one or more wireless terminals a channeldispersion measurement associated with multipath signal reception; anorthogonal frequency division multiplex (OFDM) unit configured todetermine a particular symbol length and a prefix length of OFDM symbolsbased upon the channel dispersion measurement reported to the basestation by the one or more wireless terminals; and the OFDM unit isfurther configured to send OFDM symbols having the particular symbollength and prefix length to ones of the one or more wireless terminalshaving a similar channel dispersion measurement; wherein the OFDM unitis configured to send an OFDM symbol in which the OFDM symbol isretransmitted an integer number of times in response to a determinationby the base station that a required prefix is longer than the symbol.12. The base station of claim 11, wherein the OFDM unit is furtherconfigured to select the symbol length such that the phase of thechannel can be considered to be stable.
 13. The base station of claim11, wherein the symbol length is substantially equivalent to ten timesthe prefix length.
 14. The base station of claim 11, wherein the basestation comprises a group of terminals operating collectively as thebase station.
 15. The base station of claim 11, wherein the OFDM unit isconfigured to send an OFDM symbol having a symbol duration that is shortenough that the channel is considered to be static for the duration ofeach symbol.
 16. A method comprising: a base station receiving from oneor more wireless terminals a channel dispersion measurement associatedwith multipath signal reception; the base station determining aparticular symbol length and a prefix length of OFDM symbols based uponthe channel dispersion measurement reported to the base station by theone or more wireless terminals; the base station sending OFDM symbolshaving the particular symbol length and prefix length to ones of the oneor more wireless terminals having a similar channel dispersionmeasurement; wherein the sending includes sending an OFDM symbol inwhich the OFDM symbol is retransmitted an integer number of times inresponse to a determination by the base station that a required prefixis longer than the symbol.
 17. A non-transitory computer readablestorage medium including instructions executable by a processor to:receive from one or more wireless terminals a channel dispersionmeasurement associated with multipath signal reception; determine aparticular symbol length and a prefix length of OFDM symbols based uponthe channel dispersion measurement reported to a base station by the oneor more wireless terminals; send OFDM symbols having the particularsymbol length and prefix length to ones of the one or more wirelessterminals having a similar channel dispersion measurement; and send anOFDM symbol in which the OFDM symbol is retransmitted an integer numberof times in response to a determination by the base station that arequired prefix is longer than the symbol.